Based on AJ's comment, I think this is the cost. I can confirm level 51 and 247 is correct based off my games.
Expansion 1 : 3,000 coins or 10 Airport Cash
Expansion 2 : 10,000 coins or 10 Airport Cash
Expansion 3 : 30,000 coins or 10 Airport Cash
Expansion 4 : 50,000 coins or 10 Airport Cash
Expansion 5 : 100,000 coins or 15 Airport Cash
Expansion 6 : 150,000 coins or 15 Airport Cash
Expansion 7 : 200,000 coins or 15 Airport Cash
Expansion 8 : 400,000 coins or 15 Airport Cash
Expansion 9 : 600,000 coins or 15 Airport Cash
Expansion 10 : 800,000 coins or 15 Airport Cash
Expansion 11 - 80 : 20 Airport Cash
Expansion 81 - 100: 40 Airport Cash
Expansion 101 - 120: 60 Airport Cash
Expansion 121 - 140: 80 Airport Cash
Expansion 141 - 160: 100 Airport Cash
Expansion 161 - 180: 120 Airport Cash
Expansion 181 - 200: 140 Airport Cash
Expansion 201 - 220: 160 Airport Cash
Expansion 221 - 240: 180 Airport Cash
Expansion 241 - 260: 200 Airport Cash
Expansion 261 - 280: 220 Airport Cash
Expansion 281 - 300: 240 Airport Cash
I curious if there is more info on when you can't use coins to purchase land. I'm at expansion 19 and I can purchase it for 20 cash (10 on a sale) OR 800,000. The chart seems to indicate the coin option stopping at expansion 11.